Change of Average, Local Structures for 0.5Li₂MnO₃-0.5LiMn_5/12Ni_5/12Co_1/6O₂ by Heat-Treatment under Vacuum

Abstract

We performed the heat-treatment of 0.5Li₂MnO₃-0.5LiMn_5/12Ni_5/12Co_1/6O₂ under vacuum and investigated its cathode performance, and the crystal, electronic and local structures. The coulombic efficiency of the initial cycle was improved by the vacuum heat-treatment. Based on the results of a Rietveld analysis, the Mn-ordering increased after the reductive treatments as the oxygen content and the cation-mixing decreased. From the EXAFS, it was demonstrated that the intensity of the Mn-O and Mn-M bonds of the samples changed at the discharge state of 3.3 V for the heat-treated sample under vacuum. In order to clarify the transition-metal ordering in detail, the local environment of the samples at a 3.3 V discharge was analyzed by a Pair Distribution Function analysis. Based on the analysis, it was found that the heat-treated 0.5Li₂MnO₃-0.5LiMn_5/12Ni_5/12Co_1/6O₂ under vacuum formed LiMn₆ arrangements in the transition metal layer and stabilized the local environments. The vacancies at the Li sites also increased around 3.3 V due to the vacuum heat-treatment to insert Li ions. Therefore, the reinsertion site of Li formed during the discharge process for the heat-treated sample because of the delithiation and oxygen desorption derived from the reductive heat treatment to obtain an initial enhanced discharge capacity and coulombic efficiency.